This invention relates to solenoidal electric field discharge lamps, and more specifically to means for mounting a relatively massive ferromagnetic or ferrimagnetic toroidal core within the lamp.
Solenoidal electric field lamps typically comprise a transparent evacuable envelope coated internally with a phosphor material. The envelope contains ionizable fill gas such as mercury vapor which emits ultraviolet radiation which excites the phosphor coating to generate visible wavelength radiation. The ultraviolet radiation is produced by the ionization of the fill gas which is ionized by a solenoidal electric radio frequency drive means. The drive means typically comprises a toroidal ferrite core disposed within the fill gas and coupled to a radio frequency energy source by means of windings surrounding the toroid and connected by supporting wires to a radio frequency energy source. Such lamps are described for example, in U.S. Pat. No. 4,017,764, issued Apr. 12, 1977 to John M. Anderson, the applicant herein and also in U.S. Pat. No. 4,070,602, issued Jan. 24, 1978 to Ferro et al., both of which patents are assigned to the same assignee as this application.
When operated at frequencies of approximately 50 to approximately 100 Kilohertz, the preferred core has a thickness of approximately 1.0 centimeters, an outside diameter of approximately 5.4 centimeters and an inside diameter of approximately 3.4 centimeters. A ferrite core possessing these dimensions is a relatively massive object compared with the mass of the other components in the lamp, particularly the glass envelope and the typically solid state ballast and drive circuit which operates to convert electrical energy at line frequency to electrical energy at one or more radio frequencies suitable for ionizing the fill gas. Conventional solenoidal electric field (SEF) lamp structures support the core by means of heavy gauge wires connecting the winding around the core to the energy source or ballast. (See, for example, the above-mentioned Ferro et al. patent.) If such heavy wire is used, the feedthroughs through the glass envelope prevent the use of inexpensive soda lime glass because of differences in thermal expansion coefficients. This expansion problem is solvable only by use of very expensive seals. Additionally, if the assembled lamp is to be protected from mechanical shock during shipping and operation, a more rigid support structure for the ferrite core must be provided which does not interfere with the electrical or optical performance of the lamp. Typically, lamps must be able to withstand drop tests from approximately 6 inches to a hard surface and also from approximately 6 feet to a hard surface when packed for shipment. The consequence of a weak core support may be a broken lamp or a lamp in which the core is displaced so as to have a deleterious effect on the discharge current path through the ionized fill gas. This could produce a drop in lamp efficiency with no readily discernable indication of damage.